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Predicting the reshaping of temporarily exposed bunds with XBeach-G
During coastal construction works, damages often occur to temporary constructions consisting of fine and often wide graded rock material due to exposure to waves. For an exposed breakwater core, it is important to have an idea on how much of the placed material will be moved outside the perimeter, and thus how much material needs to be rehandled before the cover layers can be constructed. For temporary defense works, it is important to know how much material needs to be placed initially in order to offer sufficient protection for a certain period. For both cases, an accurate prediction of the reshaping of such structures is necessary. The reshaping of fine and wide graded temporary structures has been simulated using the process-based model XBeach-G. The results of the model compare well with measured reshaped profiles from physical model tests. XBeach-G appears to be a suitable tool to estimate the reshaping and the resulting volume losses of rock from temporary exposed structures.
Long-term dune evolution under interacting cross-shore and longshore processes
Only few models are available that predict long-term dune evolution. Dune processes are typically modeled at shorter time scales, focusing on storm impact. Meanwhile, long-term coastline evolution models typically ignore exchange of sediment between the beach and the dune. Instead, these models often consider a fixed profile that moves seaward or landward if gradients in the longshore transport are negative or positive, respectively. As a step towards bridging the gap between nearshore, beach, and dune modelling, this study investigates the interaction between longshore transport gradients and the beach and dune evolution on decadal time scales. This aim is addressed by combining an analysis of a 22-year long data set at IJmuiden (The Netherlands) with simulations using a semi-empirical crossshore model, the CS-model.
The relative contribution of sea level rise and storm erosion to long term net coastline recession
The potential Climate change (CC) impacts on coasts and associated socio-economic and environmental risks are widely recognised internationally. One of the most talked about CC impacts is coastline recession. Any increase in mean sea level is expected to result in an upward and landward shift of the entire active profile causing net coastline recession (Bruun, 1962). Another phenomenon that can result in net coastline recession is the cumulative effect of storm erosion. This is due to the hysteresis effect in the storm erosion/dune recovery cycle (Ranasinghe et al., 2012). But what causes more recession: storms or sea level rise? This is a commonly asked question, to which science-backed answers have not been presented to date. This paper addresses this question via the application of a physics based, probabilistic numerical model at a typical swell and storm beaches located in SE Australia and The Netherlands, respectively.
Stability comparison of 9 modern placed block revetment types for slope protections
Placed block revetments are constructed to withstand the wave forces on dikes, especially in regions where rip rap is not locally available, such as the Netherlands. The blocks are placed adjacent to each other on a filter layer to form a relatively closed and smooth surface, which is easy to walk on. Large-scale test in the Delta Flume of Deltares have been carried out to compare the stability of nine types of block revetments, presently on the market in the Netherlands (2016). All tests have been performed with a comparable test setup and test program. The test program consisted of three series of tests. The first two series were short duration tests of 1000 waves with two different wave steepnesses, in which the wave height was increased step-by-step until damage occurred. The third test series was a long duration test lasting for 26 hours, or until damage occurred. The results of the tests have been used to quantify a correction factor in the calculation method. This correction factor, or stability factor, makes that the calculation method gives the same results as the Delta Flume tests, taking a safety margin into account. In this way the type-specific stability of each type of block revetment was better included in the calculation method.
Scour at the toe of rock armoured armoured structures
This paper describes the extension of the OpenFOAM numerical model to allow for exchange of sediment between the inside and outside of porous structures. This new feature has been demonstrated by an exploratory application to different toe scour cases. These cases show the sensitivity of the predicted morphological development to the hydraulic forcing conditions and toe structure dimensions. The resulting sedimentation and erosion patterns are remarkable: the significant sedimentation inside of the toe structure is contrary to expectation, especially since it seems to take place in virtually all cases. Also the scour hole on the interface of structure and sand looks unrealistically narrow.
Long-term bar dynamics using satellite imagery : a case study at Anmok Beach, South Korea
Understanding the variability of the sandbar system can therefore be crucial for informed coastal zone management. So far, the methods to study sandbar dynamics mainly include datasets of video observations or occasional bathymetric surveys. However, at most locations around the world, these types of data are not or only scarcely available. In this paper we present an alternative method to analyze long-term sandbar variability by means of freely available satellite imagery. These images are globally available since the 1980’s and, thus, have the potential to be applicable at any location in the world. Here, we will illustrate the methodology by means of a case study at Anmok beach at the South Korean East coast.
Modelling wave overtopping for grass covers and transitions in dike revetments
Transitions in the dike revetment or in the grass cover can significantly affect the wave overtopping discharge and the dike cover erosion. At the University of Twente, two PhD students recently started on the challenge of quantifying the effect of (1) waterside transition on the wave overtopping discharge and (2) transitions in grass covered dikes on dike erosion. In this paper we present their preliminary results and outline their future plans. Firstly, new laboratory experiments show that the existing wave overtopping formulas are not able to accurately predict the overtopping discharge in case of transitions on the waterside slope. Secondly, the analytical dike cover erosion model shows that transitions in grass covers significantly affect the location of maximum flow velocity and potential dike cover erosion. In future work, detailed numerical models will be developed for both the waterside slope and the landward slope to further increase our understanding of the effects of transitions on the wave overtopping discharge and the dike cover erosion.
Aged asphaltic dike revetments on (saturated) sand tested in a large Deltaflume
In the Netherlands, 600 km of the sea dikes are protected by an asphaltic revetment which must resist considerable wave loads with a significant wave height of up to 4.5 m. The subsoil is normally sandy, and the asphalt layer can fail as a result of fatigue due to repeated loading under storm conditions (Wichman & Davise 2016). Fifty years old asphalt has been taken from the Dutch Lauwersmeer dike and placed on a sand body in the large Deltaflume at Deltares, where it is possible to generate large waves (up to 4 meters). The aim of the research is to determine the resistance of the asphaltic revetment to major wave attack until failure, i.e. when it loses its function to protect the dike from erosion. The position of the phreatic line in the sand has been varied. Insights from these instrumented tests, will lead to improvement and extension of the current safety assessment.
Sensitivity analysis of a wall boundary condition for the turbulent pipe flow of Herschel–Bulkley fluids
This article follows from a previous study by the authors on the computational fluid dynamics-based analysis of Herschel–Bulkley fluids in a pipe-bounded turbulent flow. The study aims to propose a numerical method that could support engineering processes involving the design and implementation of a waste water transport system, for concentrated domestic slurry. Concentrated domestic slurry results from the reduction in the amount of water used in domestic activities (and also the separation of black and grey water). This primarily saves water and also increases the concentration of nutrients and biomass in the slurry, facilitating efficient recovery. Experiments revealed that upon concentration, domestic slurry flows as a non-Newtonian fluid of the Herschel–Bulkley type. An analytical solution for the laminar transport of such a fluid is available in literature. However, a similar solution for the turbulent transport of a Herschel–Bulkley fluid is unavailable, which prompted the development of an appropriate wall function to aid the analysis of such flows. The wall function (called y1 hereafter) was developed using Launder and Spalding’s standard wall function as a guide and was validated against a range of experimental test-cases, with positive results. y1 is assessed for its sensitivity to rheological parameters, namely the yield stress, the fluid consistency index and the behaviour index and their impact on the accuracy with which y1 can correctly quantify the pressure loss through a pipe. This is done while simulating the flow of concentrated domestic slurry using the Reynolds-Averaged Navier–Stokes (RANS) approach for turbulent flows. This serves to establish an operational envelope in terms of the rheological parameters and the average flow velocity within which y1 is a must for accuracy. One observes that, regardless of the fluid behaviour index, y1 is necessary to ensure accuracy with RANS models only in flow regimes where the wall shear stress is comparable to the yield stress within an order of magnitude. This is also the regime within which the concentrated slurry analysed as part of this research flows, making y1 a requirement. In addition, when the wall shear stress exceeds the yield stress by more than one order (either due to an inherent lower yield stress or a high flow velocity), the regular Newtonian wall function proposed by Launder and Spalding is sufficient for an accurate estimate of the pressure loss, owing to the relative reduction in non-Newtonian viscosity as compared to the turbulent viscosity.
Intertidal area disappears under sea level rise : 250 years of morphodynamic modeling in San Pablo Bay, California
In this research, we aim to validate a numerical model against observed bed level developments in San Pablo Bay, a sub‐embayment of San Francisco Estuary, to make trustworthy predictions of the estuarine bed including future sea level rise. The model included detailed tidal water movement, wind wave action, sediment transports, and resulting bed level updates. Our hindcast (1856‐1983) showed significant skill in reproducing observed bed level developments. Our forecast shows that sea level rise slowly drowns the intertidal environment. The sea level rise rate is larger than the accretion rate of the mudflats. Mitigation and adaptation strategies are required to ensure the sustainability of the estuarine environment against climate‐induced changes.